1. Field of the Invention
This invention relates to methylsiloxane compositions which cure to gels exhibiting low temperature properties. More specifically, this invention relates to compositions which are prepared in situ.
2. Background Information
The principal use intended for the gels formulated according to the present invention is as a dielectric potting gel. The use of silicone potting compounds to encapsulate electrical assemblies is well documented. U.S. Pat. No. 3,436,366 issued Apr. 1, 1969, to Modic describes how silicone materials are used to provide electrical insulation and various degrees of protection from thermal and mechanical abuse. The Modic patent also describes the possibility of formulating a dielectric potting compound using a polyorganosiloxane having only methyl and vinyl substituents. However, those substituents are arranged in such a way so as to promote a great deal of crosslinking resulting in significant tear strength. In particular, Modic teaches using an organopolysiloxane having methylvinylsiloxane and SiO.sub.2 units. Both of those units promote a high degree of crosslinking resulting in a product having very little in common with the supple gel of the present invention. Furthermore, the Modic material does not have the unique low temperature property exhibited by the gels of the present invention.
U.S. Pat. No. 3,020,260, issued Feb. 6, 1962, to Nelson is perhaps more relevant to the present invention. In Example 10 of that reference a silicone gel formulated from siloxane copolymers having only methyl and vinyl substituents is described.
More precisely, the Nelson patent teaches a method of formulating a silicone potting gel by reacting a blend of 86.95 parts by weight of copolymer (I) consisting of Me.sub.2 SiO. MeViSiO, and Me.sub.3 SiO.sub.0.5 and 8.27 parts by weight of copolymer (II) consisting of Me.sub.2 SiO, MeSiO.sub.1.5, and Me.sub.3 SiO.sub.0.5 with 4.78 parts by weight of an organohydrogensiloxane crosslinker in the presence of a platinum catalyst. The organohydrogensiloxane crosslinker described by Nelson has a formula Me.sub.2 HSiO(Me.sub.2 SiO).sub.n SiMe.sub.2 H, a viscosity of 62 centistokes at 25.degree. C. and contains 0.113 percent silicon-bonded hydrogen. The symbol "Me" represents a "--CH3" radical and the symbol "Vi" represents a "--CH.dbd.CH2" radical.
The silicone potting gel resulting from the Nelson formulation will form a dielectric silicone gel, but because of the ratios of the units employed and the presence of a unit having an internal vinyl function (MeViSiO), it will not have the structural properties and, hence, will not have the lowtemperature stability of the siloxane gel prepared according to the present invention.
Omietanski in U.S. Pat. No. 3,361,714 issued Jan. 2, 1968 discloses certain "Comb" polymers and "Star" polymers having low temperature properties. These polymers are non-random polymers and have a generic formula ##STR1## wherein R' is a monovalent hydrocarbyl radical, R" is a monovalent hydrocarbyl radical or substituted monovalent hydrocarbyl radical, p is 0 to 25, m is 1 to 21, s is greater than zero. Z' is hydrogen or R'.sub.3 Si--, and Z is hydroxyl or R'.sub.3 SiO--. Omietanski teaches that fluids and elastomers cured with organic peroxides can be obtained which exhibit low temperature properties.
Mindful of the properties and limitations of the silicone potting compounds known and described in the prior art. we endeavored to formulate a siloxane gel from a siloxane polymer having only methyl and vinyl substituents. However. these well-known units have been combined in such a manner so as to provide a novel polymeric composition which will cure rapidly to a gel at ambient or elevated temperatures and maintain its gel properties at very low temperatures. Brown et al in U.S. Pat. No. 4,374,967, issued Feb. 22, 1983, described such compositions. Brown et al describe compositions made by using a special vinyl containing polyorganosiloxane which contains dimethylsiloxane units, methylsilsesquioxane units. trimethylsiloxy units, and monovinyldimethylsiloxy units which cure to a gel which has low temperature properties, such as --120.degree. C. Brown et al prefers to crosslink the gel with a polyorganosiloxane which has two silicon-bonded hydrogens per molecule, and these hydrogen atoms are preferably on the ends of the linear polymer. Brown et al teach that, if low temperature properties are to be obtained. the special vinyl containing siloxane polymers must be used.
Lee et al in U.S. Pat. No. 4,613,659, issued Sept. 23, 1986, teach that low temperature properties can be obtained in silicone gels which are made using special siloxane polymers which contain silicon-bonded hydrogen atoms. The polymers taught by Lee et al contain dimethylsiloxane units. methylsilsesquioxane units monohydrogendimethylsiloxy units and trimethylsiloxy units.
The special polymers of Brown et al and Lee et al must be carefully prepared. The viscosity of these polysiloxanes is controlled by the ratio of the trimethylsiloxy. dimethylvinylsiloxy. or dimethylhydrogensiloxy units to the dimethylsiloxane units and the small amount of methylsilsesquioxane units present. The preparation of such polymers must be carefully done to prevent gelling during the preparation and to obtain the desired low temperature property. For example, a desirable low temperature property is obtained by polysiloxanes having at least 3 mole percent monomethylsilsesquioxane units and to prevent gelling during the preparation at least 3 mole percent trimethylsiloxy, dimethylvinylsiloxy, or a mixture of both is needed. The result is that the maximum viscosity which can be made is about 2 Pa.s. The characteristics of these gels are set when the polysiloxane is made, with respect to low temperature properties, viscosity, and many of the gel properties.